1. Field of the Invention
The invention relates in general to the removal of pollutants via biodegradation and is particularly useful in a bioscrubber application.
2. Description of Related Art
The use of biologically active materials to biodegrade pollutants has been pursued for a long time. U.S. Pat. No. 1,701,825 teaches a process for removal of "obnoxious constituents such as sulphur compounds" from a gas. The process comprises contacting a gas with a liquid medium containing bacteria. The sulfur compound is adsorbed from the gas into the liquid medium where it is oxidized by the bacteria. Preferably the gas and liquid medium are intimately mixed in a column or large shallow bed, each containing an appropriate contact material. This type of process has become known as a bioscrubber. Unfortunately, the bioscrubber systems known thus far have been too inefficient, resulting in unsatisfactory performance. At significant flow rates, multiple reactors and/or traditional chemical absorbers are frequently required. Also, large holdup reactors are frequently required in order to carry out or complete the biodegradation reaction before the liquid medium can be recycled to the gas contact zone.
For example, U.S. Pat. No. 4,544,381 (which is incorporated herein in its entirety by reference) relates to a process for removing pollutants from gas by biological means. The gas is contacted with an aqueous suspension of microorganisms for 0.5 to 5 seconds. The suspension is then retained for 3 to 10 minutes before being recycled back to the gas contact zone. Simultaneously, active biomass suspension is constantly being provided to the suspension between the contact phase and the retention phase and excess suspension is being removed.
However, the efficiency of the removal is poor. The patent states (see col. 2, lines 39-42) that "only sparingly degradable substances, such as aliphatic and aromatic chlorinated hydrocarbons, can be removed from emission gases by up to one third." To combat this shortcoming, multiple scrubber systems in series are proposed in order to improve the efficiency to around 50%. Additionally, the patent suggests post-treating the gas with a conventional trickle filter to further remove the pollutants. This embodiment is also shown in the drawings and examples. The necessity of using a filter restricts the gas flow rate in the entire system, in view of the pressure drop across the filter.
Indeed, the data in the examples show that this invention operates at low gas flow rates (about 7 to 1.8 ft.sup.3 /min. or "cfm" which, at least in the 300 mm diameter trickle filter column, translates to about 0.15 to 0.04 ft/s) and that decreasing the flow rate increases efficiency in pollutant removal. Thus, reasonable gas flow rates of the magnitude generated in commercial and industrial plants, e.g. 10,000 to 50,000 cfm or more, are not practical in this invention.
U.S. Pat. No. 5,077,025 (which is incorporated herein in its entirety by reference) relates to a process for scrubbing waste gases with an adsorbent suspension and removing the adsorbed pollutant from the suspension by biodegradation in a subsequent reactor. Specifically, the waste gas is contacted with the adsorbent suspension in an ordered mass-transfer zone to remove the pollutants from the gas. The pollutant-containing liquid is then brought to a bioreactor and contacted with bacteria for one to four hours in order to biodegrade the pollutants. Optionally, bacteria can additionally be present in the adsorbent suspension, but the patent teaches that such does not aid in removal of pollutants from the gas stream or in significantly reducing the bioreactor residence time needed.
The long bioreactor residence time (1-4 hrs.) required by this technique before the adsorbent can be reused is not convenient or desired. Additionally, the drawings indicate that countercurrent flow is contemplated in the gas contact area. But such an operation would not be practical, as plugging would occur within a matter of hours.
U.S. Pat. No. 5,279,963 (which is incorporated herein in its entirety by reference) relates to a system for decontaminating gas using a series of absorption columns and a bioreactor. The contaminant-containing gas is bubbled up through a liquid, generally water, in a packed absorption column to transfer the contaminant to the liquid. The gas and liquid are then separated and the liquid is pumped into a bioreactor comprising a packed bed and microorganisms for biodegradation of the contaminants followed by recycling of the liquid to the adsorption columns. The microorganisms may also be present in the liquid used in the absorption columns.
This system requires multiple columns and is thus inefficient. Moreover, the bubbling of the gas through the liquid restrains the gas flow rates and thus prevents the practical application of this system in commercial or industrial settings.
Another technique is described in "Control of Odor Emissions From Wastewater Treatment Plants" by Heist et al. of Kruger Inc., Denmark, published October 1995. Here a countercurrent bioscrubber packed system similar to the first stage described in U.S. Pat. No. 4,544,381 is used. The pH is controlled by the addition of certain reagents in order to improve the solubility of the target pollutant, hydrogen sulfide. However, because of the inadequacy of the bioscrub unit, a secondary carbon absorber is used. Further, inasmuch as a slurry of biologically active material is used, the scrubber becomes unstable when liquid rates approach the desired levels for commercial applications.
As indicated by the above discussion, bioscrubbers have heretofore been inadequate. The flow rates needed for real world applications have not generally been attainable unless columns and reactors of excessive size are considered. Further, the bioscrubbers have generally not been sufficiently efficient to operate alone and instead must be combined with other units. A bioscrubber that can accommodate large gas flow rates and that can provide good removal efficiencies is greatly desired by the art.